The present invention relates to a power shift transmission in which the process of shifting between different gears is adapted to be effected without interrupting the tractive power, and also to a method of operating such a transmission.
A conventional transmission of this type is described in DE 42 06 033 C2. This known transmission can be summarized as being one that is built up of two gear units in which different transmission ratios or gears are adapted to be set-up between an input shaft and an output shaft of each gear unit. Furthermore, the transmission comprises two drive shafts which are adapted to be driven by the same engine and two clutches which are respectively arranged between one of these drive shafts and the input shaft of a gear unit.
When a shift between two gears is not about to take place, one of the two clutches is open and the other one is closed and torque is transferred from the engine via the closed clutch and the gear unit attached thereto to an output drive shaft common to both gear units.
In order to effect a gear change with the known transmission, the new gear to be utilized is firstly preselected in the non-loaded gear unit, i.e. a torque-coupling connection is established in the gear unit between the output drive shaft and that part of its clutch facing the load-free gear unit. By contrast, the part of the self-same clutch on the engine side is driven directly by the drive shaft. The two parts of the clutch therefore rotate at different speeds, the ratio therebetween being determined by the transmission ratios of the currently engaged gear and the preselected gear.
In order to actually engage the preselected gear, the clutch for the as yet load-free gear unit is gradually closed so that a portion of the engine torque is transferred therethrough, whilst the clutch of the currently loaded gear unit is gradually opened at the same time so that the moment transferred by this clutch becomes smaller. Since the two clutches are never open at the same time, the gear change is effected without an interruption of the tractive power, but nevertheless slippage between the parts of the clutches inevitably occurs for as long as the shifting process persists.
Consequently, only clutches that are capable of slipping can be considered for such a transmission. The requisite size of the clutches is determined by the need for the entire engine moment to be transferred to the transmission when the clutch is closed; the two clutches thus require a considerable amount of space. Moreover, they contribute to a not insignificant degree to the costs of the transmission.